Full support of the http.FileSystem on all view engines as requested at #1575

Also, the HandleDir accepts both string and http.FileSystem (interface{}) (like the view's fs)

view/fs.go

@@ -0,0 +1,106 @@
+package view
+
+import (
+	"fmt"
+	"io/ioutil"
+	"net/http"
+	"path"
+	"path/filepath"
+	"sort"
+)
+
+// walk recursively in "fs" descends "root" path, calling "walkFn".
+func walk(fs http.FileSystem, root string, walkFn filepath.WalkFunc) error {
+	names, err := assetNames(fs, root)
+	if err != nil {
+		return fmt.Errorf("%s: %w", root, err)
+	}
+
+	for _, name := range names {
+		fullpath := path.Join(root, name)
+		f, err := fs.Open(fullpath)
+		if err != nil {
+			return fmt.Errorf("%s: %w", fullpath, err)
+		}
+		stat, err := f.Stat()
+		err = walkFn(fullpath, stat, err)
+		if err != nil {
+			if err != filepath.SkipDir {
+				return fmt.Errorf("%s: %w", fullpath, err)
+			}
+
+			continue
+		}
+
+		if stat.IsDir() {
+			if err := walk(fs, fullpath, walkFn); err != nil {
+				return fmt.Errorf("%s: %w", fullpath, err)
+			}
+		}
+	}
+
+	return nil
+}
+
+// assetNames returns the first-level directories and file, sorted, names.
+func assetNames(fs http.FileSystem, name string) ([]string, error) {
+	f, err := fs.Open(name)
+	if err != nil {
+		return nil, err
+	}
+
+	infos, err := f.Readdir(-1)
+	f.Close()
+	if err != nil {
+		return nil, err
+	}
+
+	names := make([]string, 0, len(infos))
+	for _, info := range infos {
+		// note: go-bindata fs returns full names whether
+		// the http.Dir returns the base part, so
+		// we only work with their base names.
+		name := filepath.ToSlash(info.Name())
+		name = path.Base(name)
+
+		names = append(names, name)
+	}
+
+	sort.Strings(names)
+	return names, nil
+}
+
+func asset(fs http.FileSystem, name string) ([]byte, error) {
+	f, err := fs.Open(name)
+	if err != nil {
+		return nil, err
+	}
+
+	contents, err := ioutil.ReadAll(f)
+	f.Close()
+	return contents, err
+}
+
+func getFS(fsOrDir interface{}) (fs http.FileSystem) {
+	switch v := fsOrDir.(type) {
+	case string:
+		fs = httpDirWrapper{http.Dir(v)}
+	case http.FileSystem:
+		fs = v
+	default:
+		panic(fmt.Errorf(`unexpected "fsOrDir" argument type of %T (string or http.FileSystem)`, v))
+	}
+
+	return
+}
+
+// fixes: "invalid character in file path"
+// on amber engine (it uses the virtual fs directly
+// and it uses filepath instead of the path package...).
+type httpDirWrapper struct {
+	http.Dir
+}
+
+func (d httpDirWrapper) Open(name string) (http.File, error) {
+	return d.Dir.Open(filepath.ToSlash(name))
+}